CCIE Pursuit Blog

April 4, 2008

Internetwork Expert Volume II: Lab 3 – Section 5

Exterior Gateway Routing – 16 Points

5.1 BGP Peering

This was an easy section but there were a lot of AS’s and peerings.  There are two separate AS 100.

Remember ebgp-multihop between r4 and r5.

There is no need for RR in area 100 (r1, r6, r3) as there is a full mesh.

I did miss ‘neigh 136.1.109.10 next-hop-self’ on sw3  ARGH!!!!

5.2 BGP FIltering

AS100 cannot be used a transit to reach AS54.  Configure this only on r6.

I tried to filter the traffic with an as-path access-list and a route map.  IE has a better solution: use the no-export community.

set community

(Optional) Well know communities can be specified by using the following keywords:

•internet
•local-as
•no-advertise
•no-export

r6(config-route-map)#set community ?
  <1-4294967295>  community number
  aa:nn           community number in aa:nn format
  additive        Add to the existing community
  internet        Internet (well-known community)
  local-AS        Do not send outside local AS (well-known community)
  no-advertise    Do not advertise to any peer (well-known community)
  no-export       Do not export to next AS (well-known community)
  <cr>

Nice break down of the BGP communities in the IE solution guide.  This is good because I am still shaky on BGP communities.

r3#sh ip bgp 114.0.0.0
BGP routing table entry for 114.0.0.0/8, version 25
Paths: (1 available, best #1, table Default-IP-Routing-Table, not advertised to EBGP peer)  <-
Flag: 0x880
  Advertised to update-groups:
     2
  54
    54.1.3.254 (metric 2560002816) from 136.1.136.6 (150.1.6.6)
      Origin IGP, metric 0, localpref 100, valid, internal, best
      Community: no-export

5.3 BGP Bestpath Selection

Advertise VLAN3 on r3 into BGP.  AS400 should route through AS300 to get to this prefix.  Do this configuration in AS100.

Best Path Selection Table:

Attribute Direction Applied Traffic Flow Affected Prefer
Weight Inbound Outbound High
Local_Pref Inbound Outbound High
AS-Path Outbound Inbound Shortest
MED Outbound Inbound Lowest

Before:
r5#sh ip bgp 136.1.3.0
BGP routing table entry for 136.1.3.0/24, version 27
Paths: (2 available, best #2, table Default-IP-Routing-Table)
  Advertised to update-groups:
     1          2
  300 100
    136.1.245.2 from 136.1.245.2 (150.1.2.2)
      Origin IGP, localpref 100, valid, external
  100
    136.1.15.1 from 136.1.15.1 (150.1.1.1)
      Origin IGP, localpref 100, valid, external, best

I went with MED….:-(

“To affect inbound traffic flow you must either prepend the AS-path attribute or set the MED value as the prefix is adveritised outside the AS.  However, since MED is only compared by default on prefixes learned from the SAME AS, AS-path prepending must be used in this case.”

ip prefix-list 53 seq 5 permit 136.1.3.0/24
!
route-map TASK_53_ASPATH permit 10
 match ip address prefix-list 53
 set as-path prepend 100 100
route-map TASK_53_ASPATH permit 1000
!
router bgp 100
 neigh 136.1.15.5 route-map TASK_53_ASPATH out

After:
r5#sh ip bgp 136.1.3.0
BGP routing table entry for 136.1.3.0/24, version 28
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x820
  Advertised to update-groups:
     1          2
  100 100 100
    136.1.15.1 from 136.1.15.1 (150.1.1.1)
      Origin IGP, localpref 100, valid, external
  300 100
    136.1.245.2 from 136.1.245.2 (150.1.2.2)
      Origin IGP, localpref 100, valid, external, best

5.4 BGP Attribute Manipulation

Advertise VLAN 29  on r2.

r5 should see this:

r5#sh ip bgp | i 136.1.29.0|Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 136.1.29.0/24    136.1.245.2                          100 300 i

Here’s what we currently see:

r5#sh ip bgp | i 136.1.29.0|Net
   Network          Next Hop            Metric LocPrf Weight Path
*  136.1.29.0/24    136.1.15.1                             0 100 300 i
*>                        136.1.245.2              0             0 300 i

I looks like we’re going to set the weight to 100 and filter the other route?

Attribute Direction Applied Traffic Flow Affected Prefer
Weight Inbound Outbound High
Local_Pref Inbound Outbound High
AS-Path Outbound Inbound Shortest
MED Outbound Inbound Lowest

bgp router 200
 neighbor 136.1.245.2 route-map TASK_54_WEIGHT in
!
ip prefix-list 54 seq 5 permit 136.1.29.0/24
!
route-map TASK_54_WEIGHT permit 10
 match ip address prefix-list 54
 set weight 100
route-map TASK_54_WEIGHT permit 1000

Hmmm….not quite what I got.

   Network          Next Hop            Metric LocPrf Weight Path

*  136.1.29.0/24    136.1.15.1                             0 100 300 i  <-not shown in IE
*>                             136.1.245.2              0           100 300 i

IE solution was the same as mine…kinda bad question.

5.5 BGP Bestpath Selection

YIKES!!!!

Nice breakdown though.

r2#sh ip bgp neigh 136.1.245.5 | i Cond
  Condition-map NON_EXIST, Advertise-map ADVERTISE, status: Withdraw

5.6 BGP AS Path

Advertise the EtherChannel subnet on sw3 into BGP.  Make sure that r3 and sw3 will accept BGP updates with AS100 in the AS-path.  Don’t alter r2’s configuration to accomplish this.

This is where our two AS100’s come back to haunt us.

neighbor allowas-in

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